What does Soft bounce and Hard bounce mean?

  Email Delivery


A bounce message, also known a Non-Delivery Report/Receipt (NDR) sender receives in response because of various reasons. On the top level, There can be two categories of bounces – Soft bounce and hard bounce.

When an email hard bounces, it means a permanent bounce error 5.XXX.XXX that has been returned to the sender and no further attempt to be performed to deliver email to the receipt email address. Out of many e-mail bounce types, the hard bounce is very critical and requires immediate attention by senders.

These are some standard hard bounce errors –

Unknown User / Recipient didn’t exist, No such user – This usually happens when there is no user mailbox available. There can be many reasons for such bounces like the user is moved from the company, user account deactivated, typo error in the username, etc.

Content Rejected – You can expect such responses when email subject or email body contains some spammy or restricted keywords.

Rejected by SPAM filters – These kinds of rejections happens when email subject, content, sending domain or sending IP has been red flagged by spam filters.

Blocked due to policy – Policy blocks usually happen with corporate email servers. As many corporate has own email security policies and standards enforced to met. Typically such error may occur due to missing/misconfiguration essential implementation such SPF, DKIM and DMARC at the sender side.

Unknown domain / MX record – This happens when sender email server couldn’t find the receipt domain or MX record on DNS server

Soft bounces usually mean email couldn’t be delivered for some temporary reasons, and later delivery attempt can be performed. However, you still need to monitor email addresses continuously giving soft bounces and consider them to remove from list.

Sender Reputation – Thes kinds of bounces occur when sending server IP reputation or Sending domain reputation is poor/blacklisted, which doesn’t meet the recipient email server policy or standards to qualify email delivery. In such error cases, the sender should work on reputation rebuild.

Greylisting – Sender email server is marked under greylisting rule by receipt email server.

SPF/DKIM or DMARC failure – These are the critical configurations required to be created appropriately at sender DNS server, which authenticate email server servers. Recipient email server always verifies these records to know the authenticity of an email.

Time Out – This happens when recipient email server is not reachable or no longer accepting mail to avoid overloading.

Throttling – Receipt server has received too many emails from sending IP or sending domain in a short period and will not accept further email until a grace period passes.

Error Type Meaning
421 Soft Service not available
450 Soft Mailbox unavailable
451 Soft Error in processing
452 Soft Insufficient system storage
500 Hard Address does not exist
510 Hard Other address status
511 Hard Bad destination mailbox address
512 Hard Bad destination system address
513 Hard Bad destination mailbox address syntax
514 Hard Destination mailbox address ambiguous
515 Hard Destination mailbox address valid
516 Hard Mailbox has moved
517 Hard Bad sender’s mailbox address syntax
518 Hard Bad sender’s system address
520 Soft Other or undefined mailbox status
521 Soft Mailbox disabled, not accepting messages
522 Soft Mailbox full
523 Hard Message length exceeds administrative limit
524 Hard Mailing list expansion problem
530 Hard Other or undefined mail system status
531 Soft Mail system full
532 Hard System not accepting network messages
533 Hard System not capable of selected features
534 Hard Message too big for system
540 Hard Other or undefined network or routing status
541 Hard No answer from host
542 Hard Bad connection
543 Hard Routing server failure
544 Hard Unable to route
545 Soft Network congestion
546 Hard Routing loop detected
547 Hard Delivery time expired
550 Hard Other or undefined protocol status
551 Hard Invalid command
552 Hard Syntax error
553 Soft Too many recipients
554 Hard Invalid command arguments
555 Hard Wrong protocol version
560 Hard Other or undefined media error
561 Hard Media not supported
562 Hard Conversion required and prohibited
563 Hard Conversion required but not supported
564 Hard Conversion with loss performed
565 Hard Conversion failed
570 Hard Other or undefined security status
571 Hard Delivery not authorized, message refused
572 Hard Mailing list expansion prohibited
573 Hard Security conversion required but not possible
574 Hard Security features not supported
575 Hard Cryptographic failure
576 Hard Cryptographic algorithm not supported
577 Hard Message integrity failure